Cooling for superconducting machines

ABSTRACT

A device for cooling superconducting machines has a closed thermal siphon system which can be filled with a liquid coolant and has an evaporator for evaporating the liquid coolant. In order to improve the cooling performance of the device, the surface area of the evaporator which can be wetted with the liquid coolant is increased.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to International Application No. PCT/EP2010/057098 filed on May 25, 2010 and German Application No. 10 2009 022 960.4 filed on May 28, 2009, the contents of which are hereby incorporated by reference.

BACKGROUND

The invention relates to a device for cooling superconducting machines. This device has a closed thermal siphon system which can be filled with a liquid coolant and which has an evaporator for evaporating the liquid coolant.

DE 102 44 428 A1 discloses a machine with a rotor and a stator in a machine housing which contains an installation for cooling parts within this housing. This cooling installation has on at least one face of the machine a closed system of piping with a condenser located outside the housing, with an evaporator located inside the housing and with connecting tubes running between the condenser and the evaporator, wherein the circulation of a coolant in this system is effected in accordance with a thermal siphon effect.

WO 2006/082194 A1 discloses a machine with a rotor which can rotate about an axis, the superconducting winding of which has a heat-conducting coupling, via a winding carrier and a thermal contact gas to a central cooling agent space of a stationary thermally conductive solid which projects into a hollow space in the rotor. Together with conducting parts connected to its side and a condenser space of a cooling unit located outside the machine, the cooling agent space forms a piping system in which a cooling agent circulates due to a thermal siphon effect. For the purpose of maintaining the infeed of cooling agent into the central cooling agent space even when the rotor is out of alignment, the cooling agent space is provided with a lining of a porous material, preferably a sintered material, with a high thermal conductivity.

SUMMARY

One possible object is to improve the cooling performance of a device for cooling superconducting machines.

The inventors recognized that for the purpose of achieving a required cooling performance in a device for cooling superconducting machines, it is not the absolute quantity of the liquid coolant available which is decisive but the size of a surface of the evaporator which can be wetted by the liquid coolant. The larger the surface of the evaporator which can be wetted by the liquid coolant, the more coolant can evaporate, i.e. the more thermal energy can be transferred to the evaporating coolant via this available wettable surface. Thus the available cooling performance of the device for cooling superconducting machines can be effectively raised by an enlargement of the wettable surface of the evaporator.

An evaporator is usually designed as a hollow space, the bounds of which are available as the surface of the evaporator. Depending on the level of filling with the liquid coolant, a larger or smaller surface of the evaporator is then available for evaporating the liquid coolant. In order to enlarge this surface which can be wetted by the liquid coolant, without the need to increase the quantity of the liquid coolant, it is proposed that the devices to enlarge the surface of the evaporator which can be wetted by the liquid coolant have at least one displacer for displacing the liquid coolant. With this, there is a saving on coolant combined with an enlargement of the surface of the evaporator which can be wetted by the liquid coolant.

In accordance with one advantageous embodiment of the inventors' proposals, the evaporator is arranged in the interior of a rotor of a superconducting machine. The surplus thermal energy can thereby be dissipated directly from the rotor. The enlargement of the surface of the evaporator which can be wetted by the liquid coolant, is especially advantageous with this embodiment because the volume, and with it also the surface, of an evaporator located in the interior of a rotor is normally limited by the relatively small dimensions of a rotor.

Constructional advantages are achieved in that, in accordance with another advantageous embodiment, the evaporator and the at least one displacer are cylindrical, in particular circularly cylindrical, in shape. Such shaping is simple to manufacture, and nevertheless is efficient in displacing the liquid coolant.

In accordance with another advantageous embodiment, it is proposed that the surface of the evaporator which can be wetted by the liquid coolant has a surface structure which is formed in such a way that the surface which can be effectively used for the transfer of heat is enlarged. In this manner, it is possible to achieve a particularly significant enlargement of the surface of the evaporator which can be wetted by the liquid coolant, combined with low construction costs.

Here, in accordance with another advantageous embodiment one surface structure which is particularly simple to realize, in terms of manufacturing technology, has elements which are one-dimensional, in particular groove- or fin-like.

In order to further raise the cooling performance, the surface structure has, in accordance with another advantageous embodiment, elements which are two-dimensional, in particular hole-like or spiky.

In accordance with another embodiment, the liquid coolant is neon. Neon permits a particularly favorable working point, e.g. in the cooling of high temperature super-conductors, but is however relatively expensive so that the reduction in coolant which is achieved by the inventors' proposals is particularly useful.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 a schematic diagram of a section through a superconducting machine together with a device for cooling the superconducting machine,

FIG. 2 a schematic diagram of an evaporator in accordance with the related art,

FIG. 3 an exemplary embodiment of the proposed device, with a displacer for displacing the liquid coolant,

FIG. 4 another exemplary embodiment of the proposed device, in which the surface of the evaporator which is effectively usable for the transfer of heat is enlarged, and

FIG. 5 an exemplary embodiment of the proposed device, in which use is made of various devices to enlarge the surface which can be wetted by the liquid coolant.

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

FIG. 1 shows a schematic diagram of a superconducting machine 1 together with a device for cooling the superconducting machine 1. This shows a section along the longitudinal axis of the superconducting machine 1. The superconducting machine 1 in the case of the exemplary embodiment shown in FIG. 1 is a rotating electrical machine, in particular a synchronous machine, for example a motor or a generator. This has a stator 10 together with a rotor 6. In addition, it has a housing 11 for accommodating the stator 10 and for the bearing mountings of the rotor 6. The superconducting machine 1 is cooled by a closed thermal siphon system, which has an evaporator 4, a condenser 9 together with elements which connect the evaporator 4 and the condenser 9, e.g. connecting pipes. The evaporator 4, the connecting elements and the condenser 9 form the bounds of an enclosed space, which is provided to accommodate the liquid coolant 3. The evaporator 4 has a surface 5, which can be wetted by the liquid coolant 3, via which the thermal energy arising in the rotor and which is to be dissipated is transferred to the coolant 3. In this process, the coolant 3 is normally converted from the liquid state into the gaseous state by the thermal energy transferred, i.e. the coolant 3 is evaporated or boils. Due to the lower density of the gaseous form of the coolant, it rises through the connecting elements to the condenser 9, which is at a higher geodetic level, and there it is converted back from the gaseous to the liquid state by extraction of the thermal energy which it had taken up. Due to gravity, the coolant 3 which has in this way been re-liquefied flows back to the evaporator 4, and in particular to the surface 3 of the evaporator 4 which can be wetted by the coolant 3. A cooling system of this type utilizes the so-called thermal siphon effect. The cooling circulation is maintained solely by the density differences mentioned, or gravity, as applicable.

FIG. 2 shows an axial section through the evaporator 4 of a superconducting machine with the machine stationary. The other parts of the machine are not explicitly illustrated in FIG 2. The evaporator 4 shown in FIG. 2 has a circularly cylindrical cross-section. The evaporator 4 illustrated is known from the related art. The evaporator 4 is at least partially filled with a liquid coolant 3. Here, the surface of the evaporator 4 which can be or is wetted by the liquid coolant 3 is identified with the reference mark 5.

When superconducting machines 1 are cooled using a thermal siphon system, a certain minimum area of the evaporator 4 must be wetted by the liquid coolant 3 in order to achieve the required cooling performance. Depending on the precise geometry of the evaporator 4 combined with the heat transfer, which during the cooling-down phase is frequently limited by film boiling, a comparatively large amount of liquid coolant (e.g. neon, nitrogen or similar) is required in the case of superconducting machines as presently designed.

Currently, this problem is normally solved by simply filling up with an appropriate quantity of coolant 3 to be able to wet a sufficiently large surface in a (normally horizontally arranged) cylindrical-shaped evaporator 4. In order at the same time to adhere to the concept of a closed thermal siphon system with a one-time filling, this method requires a comparatively large buffer container at room temperatures (pressurized container), in which liquid coolant 3, which gradually evaporates when the cooling system is switched off or fails, can be collected with a tolerable pressure rise. Alternatively of course, it is also possible to make allowance for the fact that a lower level of filling with coolant causes cooling-down to last longer than is really necessary.

FIG. 3 shows an evaporator 4 in an exemplary embodiment of the proposed device. The evaporator 4 is at least partially filled with a liquid coolant 3. By using an additional (advantageously cylindrical) displacer 7, the quantity of liquid required for wetting the same evaporator surface area can be substantially reduced. The device has, as the structure 7, 8 to enlarge the surface 5 of the evaporator 4 which can be wetted by the liquid coolant 3, a displacer 7 for displacing the liquid coolant 3. The displacer 7 restricts the volume available within the evaporator 4 for the liquid coolant 3, in such a way as to enlarge the surface 5 of the evaporator 4 which is actually wetted by the coolant 3.

FIG. 4 shows an evaporator 4 in another exemplary embodiment of a device in accordance with the proposals. Alternatively or additionally to the embodiment shown in FIG. 3, the functionally effective surface of the evaporator surface can itself also be substantially enlarged by the introduction of an appropriate surface structure 8. Advantageous embodiments are one-dimensional groove- or fin-like structures, with which the surfaces can in a simple way be substantially enlarged (factor 3-5). As shown in the exemplary embodiment illustrated, the structure 7, 8 for enlarging the surface 5 of the evaporator 4 which can be wetted by the liquid coolant 3 are in the form of a surface structure 8 on the surface of the evaporator, wherein the surface structure 8 is arranged so as to enlarge the surface 5 which is effectively usable for the transfer of heat. The surface structure 8 in the exemplary embodiment shown has one-dimensional elements, in this case groove- or fin-like elements. Two-dimensional variants, rather more complicated to manufacture, are also advantageous for the purpose of enlarging the surfaces (such as for example the introduction of holes or spiky structures), and permit an even greater enlargement of the effective surface.

As another exemplary embodiment, FIG. 5 shows an evaporator 4, in a device in accordance with the proposals, which has a combination of the structure 7, 8 for enlarging the surface 5 of the evaporator 4 which can be wetted by the liquid coolant 3.

In the exemplary embodiment shown in FIG. 5, the structure shown in FIG. 3, i.e. a displacer 7, is combined with the structure shown in FIG. 4, i.e. a surface structure 8 for enlarging the surface 5 of the evaporator 4 which can be wetted by the coolant 3.

The embodiments shown enable a reduction in the quantity of fluid required for wetting a particular minimum surface of the evaporator 4 as part of the thermal siphon cooling circuit. The advantages lie in the directly associated reduction in the required buffer volume (typically from several 100 liters to about one tenth of that) and thus from a smaller space requirement and lower costs. The costs of the actual filling of the thermal siphon system are also reduced thereby (less coolant 3).

In summary, the proposals relate to a device for cooling superconducting machines 1, with a closed thermal siphon system 2 which can be filled with a liquid coolant 3 and which has an evaporator 4 for evaporating the liquid coolant 3. In order to improve the cooling performance of the device, structure 7, 8 are provided for enlarging a surface 5 of the evaporator 4 which can be wetted by the coolant 3.

The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004). 

1-8. (canceled)
 9. A device for cooling a superconducting machine, comprising: a closed thermal siphon system to receive a liquid coolant; and an evaporator to evaporate the liquid coolant, the evaporator having a displacer to displace the liquid coolant and thereby increase a surface area of the evaporator which can be wetted per volume of liquid coolant.
 10. The device as claimed in claim 9, wherein the evaporator is located in an interior of a rotor of the superconducting machine.
 11. The device as claimed in claim 9, wherein the evaporator and the displacer are cylindrical, having a circular cross section.
 12. The device as claimed in claim 9, wherein the evaporator has an irregular surface structure to thereby increase the surface area of the evaporator which can be effectively used for transfer of heat to the liquid coolant.
 13. The device as claimed in claim 9, wherein the irregular surface structure is one-dimensional.
 14. The device as claimed in claim 9, wherein the irregular surface structure is formed from groove- or fin-like elements.
 15. The device as claimed in claim 9, wherein the irregular surface structure is two-dimensional.
 16. The device as claimed in claim 9, wherein the irregular surface structure is formed of hole-like or spiky elements.
 17. The device as claimed in claim 9, wherein the liquid coolant is neon.
 18. The device as claimed in claim 10, wherein the evaporator and the displacer are cylindrical, having a circular cross section.
 19. The device as claimed in claim 18, wherein the evaporator has an irregular surface structure to thereby increase the surface area of the evaporator which can be effectively used for transfer of heat to the liquid coolant.
 20. The device as claimed in claim 19, wherein the irregular surface structure is formed from groove- or fin-like elements.
 21. The device as claimed in claim 20, wherein the irregular surface structure also includes hole-like or spiky elements.
 22. The device as claimed in claim 21, wherein the liquid coolant is neon. 